This calculator calculates the price of one plank if a servant is used to cut the logs into planks. This calculator assumes you send the servant twice, once to get the logs from the bank and then again to send them to the sawmill with the logs, and that you get the maximum number of logs/planks that the servant can carry each trip. For the cost per XP, the Volatile Hammer and Sacred Clay Hammer bonuses still apply.
You need a Cook or above to make the trip to the sawmill.
It is usually cheaper to buy mahogany planks rather than getting a servant to cut them.